How To: Computing the cost of energy usage

Right now I don't have any material for a photo update, but I will later this week as I get the LED lettuce experiment re-started. For now I thought it would be interesting to share how I figure out the cost of energy used to run my bucket farm.

If you are involved in the hydroponics hobby or intend to get involved at some point, you'll need to manage energy usage to manage cost. Depending on your setup and your needs, you will be operating some combination of air pumps, water pumps, fans, lights and perhaps even heating and cooling appliances.

To get started, you need to know how much your energy company charges you for electricity. This is on your power bill and likely on your power company's website as well. This rate is usually expressed as the price per kilowatt hour of electricity used. In my case, this rate is nine cents (.09) per kilowatt hour.

You will also need to know the wattage of the device you are operating. This is easy with lamps, as they usually have the wattage printed on the bulb or base in almost all cases. With pumps, fans, etc, you may have to look at the box or see if the value is stamped on the appliance somewhere.

The formula to figure out how much it costs to operate a device for a certain number of hours is:

COST = ((DEVICE WATTAGE * HOURS USED) / 1000 ) * POWER RATE

Or, to use this formula in a practical experiment, lets say that I want to compute how much it costs me to run a 60 watt light bulb for 24 hours.

COST = ((60 * 24)/1000) * .09


Let's look at this step by step.

1. Multiply the wattage of the bulb by the number of hours it is used. 60 * 24 = 1440. This gives you the watt-hours of electricity used.

2. Divide the watt-hours by 1000 to convert them to kilowatt hours. 1440 / 1000 = 1.44 kWh

3. Multiply kilowatt hours by the price of electricity (9 cents in my case) 1.44 * .09 =0.1296

Thus, it costs me about thirteen cents (.13) to run a 60 watt bulb for 24 hours.

In addition, you can multiply this figure by 30 to figure out the monthly cost: .13 * 30 = $3.90 to operate a 60 watt bulb 24/7 for a month.

You try. Let's look at the cost of operating just my lamp. I use a 400 watt lamp and currently it runs for 13 hours a day because my peppers are now blooming and fruiting. How much does it cost me per day?

P.S. If you leave lights on around your house for other purposes, you should compute how much money you'd save by swapping your 60, 100 and 150 watt incandescent light bulbs out for 13, 26 and 42 watt compact fluorescent bulbs.